Bootstrap/scss/_alert.scss

//
// Base styles
//

.alert {
  padding: $alert-padding-y $alert-padding-x;
  margin-bottom: $alert-margin-bottom;
  border: $alert-border-width solid transparent;
  @include border-radius($alert-border-radius);
}

// Headings for larger alerts
.alert-heading {
  // Specified to prevent conflicts of changing $headings-color
  color: inherit;
}

// Provide class for links that match alerts
.alert-link {
  font-weight: $alert-link-font-weight;
}


// Dismissible alerts
//
// Expand the right padding and account for the close button's positioning.

.alert-dismissible {
  // Adjust close link position
  .close {
    position: relative;
    top: -$alert-padding-y;
    right: -$alert-padding-x;
    padding: $alert-padding-y $alert-padding-x;
    color: inherit;
  }
}


// Alternate styles
//
// Generate contextual modifier classes for colorizing the alert.

@each $color, $value in $theme-colors {
  .alert-#{$color} {
    @include alert-variant(theme-color-level($color, -10), theme-color-level($color, -9), theme-color-level($color, 6));
  }
}